1. Field of the Invention
The present invention generally relates to automatic control circuits and more particularly to an automatic control circuit for a video camera capable of automatically carrying out the centering adjustment (adjustment of the center position of a picture image) of the horizontal and vertical deflections and the adjustment of white balance.
2. Description of the Prior Art
In a video camera, for example, of the three-tube type, an automatic control circuit 10 as shown in FIG. 1 is used to automatically adjust the deflection centers thereof.
In FIG. 1, reference numerals 1R, 1G, and 1B respectively designate pick-up tubes or image sensors for red, green and blue colors. In this example, under the condition that the deflection center of the green image sensor 1G is taken as a reference, the vertical and horizontal centerings of red and blue image sensors 1R and 1B are adjusted. To this end, a deflection signal useful for the horizontal and vertical deflections is supplied from a sawtooth wave signal generating circuit 2 to the green image sensor 1G and has added thereto biases corresponding to 1H (H represents the horizontal period) in the vertical direction of the picture screen and T/2 (T&lt;&lt;H is satisfied) in the horizontal direction so that the green image sensor 1G produces a green signal Go (see FIG. 2A) which is advanced in phase by (1H+) relative to red and blue signals R and B produced from red and green image sensors 1R and 1B.
The green signal Go is supplied through a preamplifier 3G to a generating circuit 5 which generates a comparing reference signal G'o and edge signals E.sub.V and E.sub.H. The reference signal G'o (see FIG. 2B) is delayed in phase by (1H+(T/2)) relative to the green signal Go and approximately in equiphase with the red and blue signals R and B. The edge signal E.sub.V is the edge signal in the vertical direction and is used to adjust the vertical centering. This edge signal E.sub.V is formed of the green signal Go and a signal which results from delaying the green signal Go by 2H. Accordingly, the edge signal E.sub.V is a signal which has the pulse width and the polarity as shown in FIG. 2C. The other edge signal E.sub.H is used to adjust the horizontal centering.
If the vertical centering is adjusted first, one edge signal E.sub.V is first selected by a switching circuit 6 to which the edge signals E.sub.V and E.sub.H are applied and then supplied through a capacitor C.sub.1 for DC cutting to a multiplier 7. The edge signal E.sub.V is further supplied to a generating circuit 8 which generates a sampling pulse P.sub.S (see FIG. 2G).
On the other hand, the red and blue signals R and B are respectively supplied through preamplifiers 3R and 3B to a switching circuit 9. Then, the selected one of the primary color signals R and B and the reference signal G'o are both supplied to a voltage comparator 11 which detects error signals .DELTA.R and .DELTA.B corresponding to positional displacements of the red and blue signals R and B relative to the reference signal G'o. FIGS. 2D and 2E show examples of the red signal R and its error signal .DELTA.R relative to the reference signal G'o. The error signal .DELTA.R will be described hereinafter.
The error signal .DELTA.R is supplied through a capacitor C.sub.2 for DC cutting to the multiplier 7 in which it is multiplied with the edge signal E.sub.V. As a result, a multiplied output .DELTA.M.sub.R (see FIG. 2F) is produced with its pulse width corresponding to the displacement amount of the center position of the picture image and its polarity corresponding to the direction of displacement. The multiplied output .DELTA.M.sub.R is smoothed by a low-pass filter 13 and sampled and then held by a sample hold circuit 14 to which the sampling pulse P.sub.S is applied. The held output .DELTA.C.sub.R (see FIG. 2H) is supplied to a deflection correcting circuit 16R in which a vertical deflection signal applied thereto from the generator 2 is corrected and then supplied to red image sensor 1R.
Regarding the blue image sensor 1B, a held output .DELTA.C.sub.B corresponding to the error signal .DELTA.B is similarly generated and supplied to a deflection correcting circuit 16B by which a vertical deflection signal applied thereto from the generator 2 is corrected and then supplied to the blue image sensor 1B.
The horizontal centering is adjusted after the vertical centering is adjusted. At that time, the edge signal E.sub.H is selected by the switching circuit 6.
By the way, the edge signals E.sub.V and E.sub.H are formed on the basis of an object existing within a predetermined window area S.sub.W including the center of a picture screen S as shown in FIG. 3. Since the sampling pulse P.sub.S is formed on the basis of the edge signals E.sub.V and E.sub.H, if an object containing much edge components and an object containing less edge components are displaced the same distance from the deflection center, the number of samplings is different in each case and so the held outputs .DELTA.C (.DELTA.C.sub.R or .DELTA.C.sub.B) are naturally different in value.
For example, when the centering is adjusted by using the object containing less edge components, if a relation between the displacement of the centering and the held outputs .DELTA.C.sub.R, .DELTA.C.sub.B is given as shown by a straight line a in FIG. 4, such relation for the object containing much edge components becomes as shown by a straight line b in FIG. 4. Accordingly, although the displacement of the centering is indicated as X in FIG. 4, depending on the objects to be used, the held output .DELTA.C becomes considerably different for example, as indicated at .DELTA.C.sub.1 and .DELTA.C.sub.2. In that case, if the centering is adjusted correctly by using the held output .DELTA.C.sub.1, if the object containing less edge components is used, the deflection correcting amount becomes significantly insufficient so that the horizontal and vertical centering can not be adjusted correctly.
When the white balance is adjusted, the automatic control circuit 10 as shown in FIG. 1 is used to control the gains of gain control circuits (not shown) provided at the rear stages of the preamplifiers 3R and 3B. Also at that time, the object existing within the predetermined window area S.sub.W of the picture screen S is used and the level of the chrominance signal is sampled and held and the output thereof is used to carry out the balance adjustment. Therefore, under different pick-up or shooting conditions, the correcting signal for adjusting the white balance becomes sometimes different. In consequence, so long as the control system having the circuit arrangement as shown in FIG. 1 is used, a similar defect to that in the centering adjustment operation takes place.